The present invention relates to coated cemented carbide inserts with a binder phase enriched surface zone and a process for the making of the same. More particularly, the present invention relates to coated inserts in which the cemented carbide has been modified so that unique technological properties have been obtained at a given chemical composition and grain size regarding the balance between very good toughness behavior and high resistance against plastic deformation.
Coated cemented carbide inserts with binder phase enriched surface zone are today used to a great extent for machining of steel and stainless materials. Thanks to the binder phase enriched surface zone, an extension of the application area for the cutting tool material has been obtained.
Methods or processes to make cemented carbide containing WC, cubic phase (gamma-phase) and binder phase with binder phase enriched surface zones are within the techniques referred to as gradient sintering and are known through a number of patents and patent applications. According to, e.g., U.S. Pat. Nos. 4,277,283 and 4,610,931, nitrogen-containing additions are used and sintering takes place in vacuum whereas according to U.S. Pat. No. 4,548,768, the nitrogen is added in gas phase. In both cases, a binder phase enriched surface zone essentially depleted of cubic phase is obtained. U.S. Pat. No. 4,830,930 describes a binder phase enrichment obtained through decarburization after the sintering whereby a binder phase enrichment is obtained which also contains cubic phase.
In U.S. Pat. No. 4,649,084, nitrogen gas is used in connection with the sintering in order to eliminate a process step and to improve the adhesion of a subsequently deposited oxide coating.
From a fracture mechanics point of view, an enrichment of binder metal in a surface zone means that the ability of the cemented carbide to absorb deformation and stop growing cracks increases. In this way, a material is obtained with an improved ability to withstand fracture by allowing greater deformations or by preventing cracks from growing, compared to a material with mainly the same composition but homogeneous microstructure. The cutting material, thus, obtains a tougher behavior.
When gradient sintering according to the known technique of vacuum sintering of a nitrogen-containing cemented carbide, the nitrogen is usually added by adding a small amount of nitrogen-containing raw materials. Due to the fact that the nitrogen activity in the furnace atmosphere at the sintering is below the average nitrogen activity in the cubic phase, the nitrogen-containing cubic phase will give off nitrogen through the liquid binder phase to the furnace atmosphere. There is a certain disagreement about the kinetics in this dissolution process. The opinion seems to be that when the nitrogen leaves, this generates conditions for a complete dissolution of the cubic phase in the surface zone of the material. The process is thought to be controlled by diffusion of nitrogen and by diffusion of the metal components of the cubic phase. Regardless, the result is that the volume which previously was occupied by the cubic phase after its dissolution is occupied by liquid binder metal. Through this process, a binder phase enriched surface zone is created after the solidification of the binder phase. The metal components in the dissolved cubic phase diffuse inwardly and are precipitated on available undissolved cubic phase present further in the material. The content of these elements therefore increases in a zone inside the binder phase enriched surface zone at the same time as a corresponding decrease in the binder phase content is obtained.
A characteristic distribution of Co, Ti and W as a function of the distance from the surface of a cemented carbide with binder phase enrichment obtained through the above-mentioned process appears, e.g., from FIG. 1 in U.S. Pat. No. 4,830,930. Outermost, there is a surface zone enriched in binder phase and completely or partly depleted of cubic phase. Inside this surface zone there is an area with an enrichment of the metallic element(s) present in the cubic phase, in particular Ti, Ta and Nb, and where the binder phase content is considerably lower than the average content of binder phase in the interior of the cemented carbide body. The decrease in binder phase content for cemented carbide with about 6 weight-% cobalt and 9 weight-% cubic phase can be up to about 2 weight-%, i.e., a relative decrease of the order of 30%. Cracks grow easily in this zone, which has a decisive influence on the fracture frequency during machining when the cemented carbide body is used as a metal-cutting insert.